Pressurized product delivery systems

ABSTRACT

An apparatus for dispensing pressurized material is comprised of a container, a polymeric matrix having a network of macropores located within the container, and a propellant that may be reversibly sorbed with the network of macropores so as to desorb as the partial pressure of the propellant within the container decreases. The network of pores is substantially non-swellable upon sorption and desorption of the propellant during filling and use of the dispensing apparatus. The polymeric matrix may comprise aggregates of macroporous polymeric particles in which the particles define a substantially non-collapsible pore network and the aggregates define interstitial spaces between the particles, and the interstitial spaces are substantially larger than the pore network.

BACKGROUND OF THE INVENTION

The present invention relates generally to systems for storing anddispensing gases, liquids, and/or solids. More particularly, the presentinvention relates to pressurized delivery systems and materials andmethods for producing such systems.

Pressurized systems are widely used for dispensing a variety of consumerand industrial products including shaving cream, non-stick cookingproducts, hair spray, insect repellant, spray paint, and a wide varietyof cleaning materials. These systems, which are usually referred to as"aerosols," commonly include three components: (1) a product to bedispensed, (2) a propellant, and (3) a pressurized container including avalve actuator (commonly a push button). The container is typicallycylindrical, usually being fabricated as a sheet metal can capable ofwithstanding moderate pressures. In operation, a push button or otheractuator opens the valve which will allow the product to be expelledfrom an opening or nozzle as a wet spray, fine spray, powder, foam orpaste depending upon the application. The propellant, which is typicallya gas under ambient conditions, is in most cases expelled from thecontainer with the dispensed product.

The propellant and dispensed product combination may take the form of asolution, emulsion or powder within the aerosol container. In asolution, the propellant is at least partially miscible in the product.In an emulsion, the propellant forms an internal phase which vaporizeson discharge. Shaving cream is a product commonly sold with anemulsion-based dispenser system. Powder systems require a dispersion ofthe product in the propellant and must be specially designed to avoidclogging of the valve or nozzle during discharge.

As the above examples suggest, in most applications the propellant gasis intimately mixed with the product. In some systems, however, thepropellant gas is separated from the product by a mechanical barriersuch as a piston or diaphragm which prevents the propellant from passinginto the product. These systems are described in European PatentSpecification EP0089971 which is incorporated herein for all purposes.

A number of practical considerations limit the substances which can beused as propellant gases and/or the circumstances in which a givensubstance can be used as a propellant gas. Traditionally, these haveincluded the ability to sustain pressure within acceptable limits duringuse, safety factors such as the flammability and toxicity of thepropellant, and chemical reactivity of the propellant with the containerand, mainly in the case of non-barrier dispensers, reactivity of thepropellant with the product to be dispensed. For some applications, theflammability provides a particular problem. For example, many non-stickvegetable oil products will be used near an open gas flame or hot stove.Today, the environmental impact of the propellant must also beconsidered.

The principal advantage of liquefied propellant is their ability toprovide a constant pressure within the aerosol container throughout thelife of the product. Chlorofluorocarbons ("CFC"s) and low molecularweight hydrocarbons such as propane, butane and isobutane are the mostcommon liquefied propellants. Because these compounds are liquid at themoderate pressures of aerosol containers, the internal pressure of thecontainer is determined by the propellant's vapor pressure. As long asthe propellant is present in liquid form within the container (typicallythe entire life of the product), the pressure will remain nearlyconstant and the product will be dispensed in the same condition.Consumers today expect this performance from aerosol containers.

Unfortunately, the liquid propellants in widespread use are eitherenvironmentally destructive or flammable and toxic. Evidence is mountingthat CFCs are destroying the protective ozone layer high in theatmosphere, thus permitting dangerous levels of ultra-violet radiationto reach the earth surface. In response, the countries of the world havetaken steps to curtail the use of CFCs and other commonly usedpropellants. Thus alternative dispensing systems will need to bedeveloped.

One alternative to freon and hydrocarbon propellants is compressed"safe" gas propellants such as nitrous oxide, nitrogen, and carbondioxide. Unlike the liquid propellants, these gases are nontoxic, low incost and quite inert. Unfortunately, they can not be liquefied atmoderate pressures. Thus, the internal pressure of "safe" gas aerosolcontainers quickly decreases as the gas--and hence the product--aredischarged. The rate at which the product is dispensed and theconsistency (whether it be foam, aerosol spray, or liquid) will thusvary over the life of the product. Hence these products are unacceptablefor most consumer uses.

As a partial solution to this pressure decay problem, liquid solventscapable of dissolving the safe gas have been included within thedispenser. In theory, the dissolved gas should come out of solution asthe vapor phase gas is discharged, thus maintaining an even pressurethroughout the product's life. Ethanol may, for example, be used as asolvent when cosmetic products are to be dispensed, and acetone orpetroleum distillates may be used when insecticides or paints are to bedispensed. However, these applications are generally limited because thepropellant is often relatively insoluble or otherwise incompatible indispensed product.

Another attempt to employ safe gas propellants in a pressurizeddispensing apparatus is described in European Patent Application 385 773which is incorporated herein by reference for all purposes. According tothat application, carbon dioxide or other gaseous propellant isstored--alone or with a liquid solvent--in a "swellable" polymericmaterial contained within an aerosol dispenser. The propellant is heldin microvoids located between the individual molecules of the polymericmaterial. Thus, the polymeric material acts as a reservoir for thepropellant gas, allowing greater amounts of gas to be stored within apressurized container. As the container is discharged and the internalpressure begins to drop, the polymeric material will release the storedgas, mitigating the pressure loss, and, in theory, provide more uniformproduct properties over the life of the container. Unfortunately, thepolymeric materials discussed in EP 385 773 swell as the container ischarged, allowing only a limited amount of propellant to be sorbedbefore the available container space is completely occupied by thepolymer/propellant composition. And on discharge, the volume of thepolymer decreases resulting in less head space, less available pressureand, less efficient dispensing near the end of the product's usefullife.

Accordingly, there exists a need for improved pressurized dispensingapparatus which have enhanced capacity for propellants, particularly"safe" propellants as described above, but which do not suffer from thedisadvantages noted above. In particular, the dispensing apparatusshould employ a reservoir which does not expand to fill a major portionof the dispenser volume, and the dispenser should not exhibitsubstantial pressure decay during the life of the product.

SUMMARY OF THE INVENTION

The present invention provides improved pressurized dispensingapparatus, compositions, and methods for using the apparatus. Theapparatus will employ a "reservoir" of gas propellant reversibly sorbedwithin a substantially non-collapsible pore network defined by a matrixof rigid macroporous polymeric particles within a container. Bymaintaining such a reservoir, the propellant will be released within thecontainer as the partial pressure is depleted through use. Thus, thepresent invention is able to employ a wide variety of propellantsincluding those which are not liquids at room temperature and moderatepressure, and particularly including safe propellants as describedabove.

The non-collapsible particle porous matrix of the present invention hassignificant advantages over the swellable matrices described above. Inparticular, the matrices of the present invention will not expand tofill the container as the propellant is added to the apparatus and willnot shrink to increase the headspace in the container during normal use,i.e. discharge. Thus, a higher pressure can be maintained over thecourse of the product's life. In addition, some swellable polymers aremade from particularly hazardous materials. For example, polyurethanecross linking is employed in some swellable hydrogels (see EP 385 773)which will give off hydrogen cyanide gas when burned.

One aspect of the present invention relates to compositions of matterfor storing a propellant under pressure, particularly within adispensing container. These compositions comprise substantially rigid,crosslinked, polymeric particles that contain a network of macropores.The propellant and, in many cases, a solvent capable of dissolving thepropellant are stored within the network of macropores in a manner suchthat the propellant is reversibly sorbed (i.e. readily desorbed when thepressure decreases). Preferably the polymeric material will be acopolymer of ethylenic monomers. A particularly preferred example is acopolymer of methyl methacrylate and ethylene glycol dimethacrylate.Further, the polymeric particles will preferably have a porosity of atleast 30%. Particles with higher porosities are preferred because theyhave a greater void volume and surface area per unit volume, and hence agreater capacity for storing propellant. In many applications, a solventmay be added to the system to further increase the propellant storingcapacity. Preferred solvents will dissolve substantial amounts ofpropellant. When a solvent of this type is employed, the rigid polymericmaterials should be able to take up at least 100% of their own weight insolvent.

Another aspect of the present invention is a pressurized dispensingapparatus comprising a container with a valve, a matrix of substantiallyrigid macroporous polymeric particles defining a substantiallynon-collapsible pore network contained within the container, and apropellant reversibly sorbed within the pore network, whereby thepropellant pressurizes said container. The rigid polymeric particlesand/or pellets of this invention may be combined, together with apropellant and a product, within a container to form a pressurizeddispenser. Typically, the dispenser valve can be controlled by apushbutton or other actuator commonly found on commercial aerosolcontainers. When the valve is opened by a user, some of theproduct--which may be any of a variety of consumer and industrialmaterials--will be expelled from the container, driven by the propellantpressure. The propellant will in some cases be expelled with theproduct, but in preferred embodiments it will remain confined within thecontainer, as by a piston or flexible diaphragm. The container pressureshould not drop below about 15% of its initial value throughout thecourse of the product's life, and, preferably, will not drop below about20% of its initial value. Most preferably, the pressure will not dropbelow 25% of its initial value. Further, the pressure should notdecrease to less than about 90%, and preferably not less than about 80%,of its initial value until more than 50% of its total propellant hasbeen expelled.

A preferred rigid macroporous polymeric matrix for storing propellantunder pressure is in the form of aggregates of macroporous polymericparticles (sometimes taking the form of hard pellets or tablets).Preferably, the pellets will be between about 5 mm and about 25 mm indiameter and weigh between about 0.1 and about 2 grams. The particleswill define a substantially non-collapsible pore network (as describedabove) with the aggregate further defining interstitial spaces betweenthe particles such that the interstitial spaces are substantially largerthan the pore network. Thus, the interstitial spaces will notsubstantially limit the rate of propellant sorption and desorption fromthe macroporous network. Further, the pellets will preferably have ahardness of at least 4 kgs as measured by a Penwalt Stokes tablethardness tester to resist crumbling during transportation or manufactureof the dispenser. In some cases, the aggregates may be at least bepartially held together by a binder.

A preferred process for preparing a pellet aggregate for use inpressurized dispensing containers, includes the following steps: (1)mixing substantially rigid crosslinked polymer macroporous particleswith a binder to form a mixture of polymer and binder; and (2)compressing the mixture to form a pellet, wherein said particles definea substantially non-collapsible pore network and the pellet definesinterstitial spaces between the particles such that the interstitialspaces are substantially larger than the pore network.

Another aspect of the present invention is a method for filling adispenser with a pressurized material. The method includes the followingsteps: (1) introducing to a container a matrix of rigid macroporouspolymeric particles; (2) introducing to the container a propellant thatmay be reversibly sorbed within the macroporous polymeric particles; and(3) introducing to the container a product to be dispensed.

Another aspect of the present invention is a method for dispensing apressurized material from a container having a valve. The processinvolves opening the valve to permit the pressurized material to exitthe container, wherein the container contains a matrix of rigidmacroporous polymeric particles having a network of macropores with apropellant reversibly sorbed within the network of macropores. Thepolymeric particles will preferably have a porosity and a rigidity suchthat the pressure within the container will not decrease by more thanabout 80% until 50% of the propellant has been dispensed from thecontainer.

The present invention employs a simple and inexpensive method for makinga variety of gas propellants available for use in pressurized dispensingsystems. Other features and advantages of the invention will appear fromthe following description in which the preferred embodiment is set forthin detail in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a preferred dispensing apparatus according to the presentinvention.

FIG. 2 is a graph of pressure versus depth of discharge for a dispenseremploying a carbon dioxide propellant, acetone and a preferredmacroporous polymeric matrix according to the present invention.

FIG. 3 is a graph of pressure versus depth of discharge for a dispenseremploying a carbon dioxide propellant, acetone and a different preferredmacroporous polymeric matrix according to the present invention.

FIG. 4 is a graph of pressure versus depth of discharge for dispenserpressurized with carbon dioxide propellant only.

DESCRIPTION OF THE PREFERRED EMBODIMENT General

The present invention relates to compositions and associated apparatusand methods for storing and releasing a gas or propellant undercontrolled conditions. This invention employs certain aspects of newaerosol container technologies and macroporous polymeric particletechnologies to provide a dispensing system having relatively constantdischarge characteristics. In preferred embodiments, a non-toxic,environmentally safe gas can serve as a propellant in an aerosoldispenser used to dispense various consumer and industrial products.

Dispensers of the present invention employ propellant reservoir matricescomprising substantially rigid, crosslinked polymeric particles whichare preferably formed as beads. The matrices will provide a macroporousnetwork with pore diameters on the order of micrometers. The polymericparticles of this invention will preferably have a high porosity andsubstantial rigidity. These and other properties can be controlled bythe polymer formation reaction conditions as described in detail below.

The propellant is maintained within the macropores in an amount thatwill depend upon the partial pressure of the surrounding gas. As thepartial pressure of the gas decreases (as is typical during discharge ofa gas propellant from an aerosol dispenser), more gas will be freed fromthe porous network to minimize the loss of partial pressure within thecontainer. Because the polymeric matrix provides a high surface areareservoir for the propellant, more gas can be stored in the pressurizedcontainer and more product can be dispensed. And because the polymericmatrices of the present invention are rigid, they will not use upadditional space as propellant gas is added to the system. Conversely,as the system is discharged during dispensing, the polymer matrix willnot shrink to create more head space. Thus, the macroporous polymericmatrices of the present invention will be better able to maintain highpressures during use of the dispensing apparatus.

In some embodiments of this invention, the polymeric material will beused in conjunction with a solvent for the propellant. Typically, thesolvent will permit greater quantities of the gas to stored for use indispenser. Preferred polymeric particles will be able to absorb at leasttheir own weight in solvent. Some polymeric particles of this inventionwill typically be able to absorb 1000% and often as much as 1500% ormore of their own weight in solvents.

The polymeric particles of this invention are preferably aggregated or"pelletized" to facilitate handling during the manufacturing process.Particles of light, non-aggregated powders are more difficult to add tothe container than heavier pellets during assembly of the dispensingapparatus. Relatively fewer pellets need to be added to a container. Inaddition, expensive filtration systems for removing dust from the airmay be necessary in manufacturing facilities where light powderyparticles are employed. Such filtration systems are not necessary whenpellets are used in manufacturing. The pellets formed according to thepresent invention are preferably sufficiently hard and crumble resistentto withstand transportation and manufacturing procedures. They may beformed by standard wet or dry granulation techniques employing standardpelletizing machinery. In general, pelletized polymeric particles arepreferred because they are more economical to use in the dispensermanufacturing process.

Relative to the unpelletized macroporous polymeric particles, pelletswill often have a slightly reduced capacity for the propellant andsolvent because of the presence of some binder. In addition, thepropellant must be transported by diffusion or some other process to andfrom the interior of the pellet. This effect, however, is minor becausepellets are so that the interstitial spaces between the individualparticles are substantially larger than the macropores (where thepropellant is stored) within the individual polymer particles. Further,pelletized polymers still have enough propellant capacity to performsatisfactorily. Thus, any disadvantage associated with pelletizing ismore than offset by the manufacturing advantage.

Definitions

Certain terms used herein are intended to have the following generaldefinitions:

"Reversibly sorbed" refers to a material such as a propellant gas or asolvent that is absorbed or adsorbed or otherwise taken up by anothermaterial, typically a solid or liquid, such that the sorbed material istaken up as its partial pressure increases and released as its partialpressure decreases.

"Substantially rigid" refers to a material such as a macroporous polymerparticle that will not change volume by more than 10% during a givenprocess such as absorbing a propellant or solvent. Preferably, thematerial will not swell by more than 5% in volume, and most preferablynot more than 1% during a process. For a powdered or granular material,the volume change will be determined with respect to individualparticles rather than a collection or agglomeration of such particles.

"Macropore" describes a pore having an average diameter of between about0.001 and 1 micrometers. These are only average diameters, and a givensample may have a much wider distribution of pore sizes. For example,some pores found in the polymers of the present invention may be assmall as 0.0001 micrometers and some may be as large as 3 micrometers.The average values for the diameter account for various pore geometries,as most macropores are not truly cylindrical. Macropores will have awide range of shapes, lengths and tortuosities that deviate to varyingextent from an ideal cylinder. A macropore should be distinguished froma microvoid which refers to the interstices between individual or smallgroups of molecules. In most instances, substantially less material suchas a solvent or propellant can be stored in an individual microvoid thanin a macropore.

The Dispensing Apparatus

FIG. 1 shows a preferred dispensing apparatus of the present invention.The dispensing apparatus 10 includes a rigid, pressure-resistantcontainer 12 having a flexible bag or diaphragm 14 (which alternatelymay be in the form of a piston) disposed within the container interior.The bag 14 contains a product 24 which is to be dispensed from thecontainer through the container nozzle 18. Below the nozzle is a valve16 which controls the flow of product 24 from container 12 throughnozzle 18. The valve is opened and closed by a valve actuator 26 whichis activated from the outside of container 12. A rigid macroporouspolymeric matrix 2 together with a sorbed propellant (not shown,typically a gas stored within the polymeric matrix) are contained withina propellant chamber 22, the upper surface of which is defined by bag14. Thus, polymeric matrix 20 is separated from and never directlycontacts the product to be dispensed 24. The bag may be permeable to thepropellant, in which case the product and propellant will contact oneanother and the propellant will be expelled from container 12 with theproduct.

A variety of container shapes, sizes, and construction materials may beemployed with the present invention, but preferred containers will beable to withstand pressures of up to about 200 pounds per square inch ofpressure, with most preferred containers withstanding pressures of up toabout 300 pounds per square inch. The valve actuator may be apushbutton, a dial, a lever, a sliding button, or any other meanssuitable for manual operation. The nozzle will typically form the top ofthe container, but may be located in other positions of the containersuch as the side. The nozzle may be replaced by other suitable forms ofproduct conduit such as a pipe.

To operate the dispensing apparatus 10, the valve 16 is opened by valveactuator 26 permitting unrestricted flow of the product out of thecontainer 12 through nozzle 18. The propellant pressurizes the containersuch that the pressure inside the container is higher than the pressureoutside the container. Because, the propellant is at least partiallyblocked by bag 14 from leaving the container, the propellant will exertpressure on the side of bag 14 opposite product 24. Thus, when valve 16is opened the propellant will press the product out of bag 14 past valve16 and out nozzle 18. As the product 24 is dispensed, the propellantwill either leave with the product, fill the space previously occupiedby bag 14 and the product, or do both. The macroporous polymeric matrix20 contains a network of pores that serve as a reservoir for thepropellant (and sometimes a solvent for the propellant). Because thepropellant is reversibly sorbed within the network of pores, it will bedesorbed as the partial pressure of the propellant decreases within thecontainer interior. Thus, the pressure in container 12 will not decreaseas rapidly as if there were no reservoir for the propellant. Preferably,the polymeric particles will have a porosity and a rigidity such thatthe pressure within the container will not decrease by more than about80% until 50% of the pressurized product has been dispensed from thecontainer.

The dispenser of FIG. 1 is a "barrier-type" dispenser with a flexiblebag surrounding the product. Another type of barrier-type dispenserincludes a sliding piston between the product and the propellant. Suchdispenser includes mostly the same components as the bag-type dispenser,except that the bag is replaced by a piston. Thus, the propellant exertsforce on a piston rather than a bag, and during discharge, the piston ismoved toward the nozzle. The piston will form a substantially leak-tightseal with the container wall to separate the propellant from the productto be dispensed. Thus, in a manner similar to the flexible bag of thebarrier-type dispenser shown in FIG. 1, the piston of this embodimentmaintains the dispensable product out of direct contact with themacroporous polymeric matrix while transmitting the pressure of thepropellant gas to the dispensable product for controlled release throughthe valve. The piston-type dispenser does not release propellant gasinto the atmosphere in normal operation.

In other embodiments, the product will not be separated from thepropellant and polymeric matrix by a bag, piston, diaphragm or othermeans. In these embodiments, the propellant, product, and polymericmatrix will be present in a common chamber, and the propellant will bedispensed along with the product. These systems will typically containmany components in common with the bag-type dispenser described above.Such barrier-free embodiments may further comprise a screen, filter, orthe like, to prevent plugging of the valve and nozzle by the polymericparticles or aggregates.

During manufacture, the propellant, macroporous polymeric matrix andsolvent, if any, must be added to the dispenser container. It isimportant that the correct amount of propellant is placed in thecontainer before the propellant chamber is sealed. Too little propellantwill result in insufficient internal pressure to effectively dispensethe product, whereas too much propellant will result in excess internalpressure with a consequent danger of the dispenser bursting. If thepropellant is carbon dioxide or other gas that can easily be condensed,the amount of propellant to be added to the container can be determinedby simply providing the propellant in the form of a liquid or relativelyuniform pellets of frozen material (e.g. dry ice). As for the solvent,it can be easily metered by simple volumetric measurement, or by directweighing.

However, since the solid or liquid propellants will likely have a verylow temperature (around -80 degrees Centigrade for dry ice), theirsimple addition to the solvent at ambient temperature will lead to rapidvaporization of the propellant, and consequent loss of propellant gasinstead of its necessary sorption in the solvent. One way to mitigatethis loss is by first chilling the solvent (acetone may be used with dryice) to near the temperature of the condensed or sublimed propellant.For example, the solvent can be first chilled to approximately thetemperature of the liquefied or solidified propellant. This procedure isdescribed in more detail together with three other procedures in EP 385773. These procedures will be briefly discussed below.

The first procedure is employed with a dispenser of the type describedin European Patent Specification EP 0089 971. In this procedure, thepropellant chamber is accessible at the lower end of the dispenser by afiling hole sealable by a plug. The requisite quantity of gas-freeliquid solvent is poured through the filling hole into the propellantchamber while the dispenser is inverted. Then the appropriate quantityof liquefied or pelletized propellant (e.g. 1 or more pellets of carbondioxide) together with the polymeric matrix (preferably pelletized) isadded through the filling hole and the plug is immediately inserted toseal the propellant chamber. If the plug is immediately applied,relatively little propellant gas will be lost by vaporization andventing. The sealed dispenser may then be agitated to assist sorption ofthe rapidly vaporizing propellant in the solvent. The risk of thisprocedure lies in a probable over-pressurization of the dispenser in theinterval between vaporization and sorption of the propellant with aconsequent risk of the dispenser bursting. Further, control of resultantpropellant pressure may be difficult because of the time-critical natureof the procedure.

The second procedure is a modification of the first procedure in thatthe pellet(s) of carbon dioxide (or other solidified propellant) arewrapped in a small piece of paper or other suitable material ofrelatively low thermal conductivity prior to being dropped through thefilling hole into the propellant chamber. The wrapping material may besoluble or insoluble in the acetone or other liquid solvent(s) employed.The paper acts as a thermal barrier which delays vaporization of thefrozen propellant by contact with the relatively hot (ambienttemperature) solvent, allowing more time in which to add the macroporouspolymeric particles or pellets and insert the plug into the filling holebefore significant loss of propellant gas occurs. The small piece ofpaper remains in the sealed propellant chamber but does notsignificantly or adversely affect the normal operation of the dispenser.

In the third procedure (described above), the carbon dioxide or otherpropellant is added to the solvent while the solvent is outside thepressurized dispenser. Premature vaporization of the propellant isobviated by pre-chilling the solvent to approximately the temperature ofthe subsequently added propellant. The macroporous polymeric particlesor pellets are added contemporaneously with the solvent and propellant.According to EP 385 773, a batch process was employed in whichapproximately 10 milliliters of liquid acetone was chilled to atemperature of about -80 degrees Centigrade (comfortably above thefreezing point of commercial-purity acetone). A carbon dioxide pelletwith a nominal weight of 1.5 grams was then added to the pre-chilledacetone. The thermal interaction of the acetone with the carbon dioxidewas minimal since both substances had approximately equal temperatures.Moreover, the adsorption of a small quantity of carbon dioxide intoacetone at a temperature of -80 degrees Centigrade would take placeinstantly. The resultant carbon dioxide/acetone composite was thentransferred into the propellant chamber of the dispenser containerbefore significant warming took place, and the propellant chamber waspromptly sealed. When the temperature of the dispenser stabilized atambient indoor temperature ("room temperature"), the dispenser was fullypressurized within acceptable tolerances for initial pressurization, andready for use.

The fourth procedure is similar to the third procedure in that theacetone or other solvent is pre-chilled to a predetermined temperature,but differs in that the carbon dioxide or other propellant is added as agas to the pre-chilled solvent. Thus, the gas is pre-dissolved in thesolvent, and only the solvent and polymeric material need be added tothe dispenser container. According to EP 385 773, -55 degrees Centigradehas been established to be the exact temperature at which the acetoneshould be maintained while gaseous carbon dioxide is bubbled through theacetone. This will permit the acetone to absorb the correct proportionof carbon dioxide for subsequent use as a propellant in a standardpiston-barrier pressurized dispenser as manufactured and sold byRocep-Lusol Limited of Great Britain. Absorption of carbon dioxide inacetone at that temperature is quickly achieved. The liquid mixture ofcarbon dioxide and acetone at -55 degree Centigrade is then transferreddirectly into the dispenser container. Only when the temperatureincreases from -55 degrees Centigrade does carbon dioxide start to boiloff. This temperature control is therefore a way of accurately meteringthe volume of carbon dioxide required for a given volume of acetone.

Polymeric Material

Polymeric particles useful in the present invention each define anetwork of internal pores open to the exterior of the particle, whichpores are capable of reversibly sorbing a propellant and optionallyother fluid of interest. The particles will be substantially rigid sothat the pore network is non-collapsible, preferably being formed fromhighly crosslinked copolymers as described in detail below. Inparticular, the polymeric particles will be non-swellable in thepropellant and solvent used in the delivery system, thus avoiding theproblems with swellable matrices described above.

The rigid polymer particles of the present invention will have greaterthan 5% cross-linking, usually having in the range from about 10% to100% cross-linking, more usually having in the range from about 20% to95% cross-linking, and typically being in the range from about 25% toabout 90% cross-linking. The calculated or theoretical percentage ofcross-linking is defined as the weight of unsaturated monomer (ormonomers) containing more than one unsaturated group divided by thetotal weight of monomer, including both mono- and poly-unsaturatedmonomers.

Polymeric particles suitable for use in the present invention willusually be non-toxic. Most particle preparation processes will result inthe formulation of spherical beads, but beads having non-sphericalasymmetric, and/or irregular geometries will also find use so long asthey meet the necessary physical parameters set forth below. Suitablepolymeric particles will not readily undergo unwanted reactions, will bestable over a wide pH range, and will resist moderate oxidation andreduction. The particles should be stable at lower as well as highertemperatures and have a relatively long shelf life. Desirable physicalparameters for the polymeric particles are as follows:

    ______________________________________                 Broad Range                           Preferred Range    ______________________________________    Particle Size  1-500 μm 2-150 μm    Particle Density                   0.1-2.0 g/cc                               0.2-1.5 g/cc    Pore Volume    0.5-6.0 cc/g                               1.0-5.0 cc/g    Avg. Pore Diameter                   0.001-3 μm                               0.003-1 μm    Surface Area   1-500 m.sup.2 /g                               20-200 m.sup.2 /g    ______________________________________

Following conventional methods of measuring and expressing pore sizes,the pore diameters are measured by techniques such as nitrogen ormercury porosimetry and are based on the model of a pore having acylindrical shape.

The particles may be formed from a wide variety of polymers, includingpolyvinyl alcohol, polyethylene, polypropylene, polystyrene,polyacrylamide, polyether, epoxy, ethylene vinyl acetate copolymer,polyvinylidene chloride, polyvinyl chloride, polyacrylate,polyacrylonitrile, chlorinated polyethylene, acetal copolymer, polyvinylpyrrolidone, poly(p-xylene), polymethylmethacrylate, polyvinyl acetate,and polyhydroxyethyl methacrylate. Preferably the polymers will havesome degree of crosslinking. Thus, the above polymers will only rarelybe employed in pure form. More commonly, copolymers will be formed thatinclude monomers used to produce the above polymers.

Thus, at least one monomer should be polyethylenically unsaturated, andusually the polymer will include a monoethylenically unsaturatedco-monomer. The degree of cross-linking may then be controlled byadjusting the ratio of monoethylenically unsaturated monomer topolyethylenically unsaturated monomer, as discussed in more detailhereinbelow.

Monoethylenically unsaturated monomers suitable for preparing polymerparticles for the polymer delivery system include ethylene, propylene,isobutylene, diisobutylene, styrene, ethylvinylbenzene, vinyltoluene,and dicyclopentadiene; esters of acrylic and methacrylic acid, includingthe methyl, ethyl, propyl, isopropyl, butyl, sebutyl, tert-butyl, amyl,hexyl, octyl, ethylhexyl, decyl, dodecyl, cyclohexyl, isobornyl, phenyl,benzyl, alkylphenyl, ethoxymethyl, ethoxyethyl, ethoxypropyl,propoxymethyl, propoxyethyl, propoxypropyl, ethoxyphenyl, ethoxybenzyl,and ethoxycyclohexyl esters; vinyl esters, including vinyl acetate,vinyl propionate, vinyl butyrate, vinyl stearate and vinyl laurate;vinyl ketones, including vinyl methyl ketone, vinyl ethyl ketone, vinylisopropyl ketone, and methyl isopropenyl ketone; vinyl ethers, includingvinyl methyl ether, vinyl ethyl ether, vinyl propyl ether, and vinylisobutyl ether; and the like.

Polyethylenically unsaturated monomers which ordinarily act as thoughthey have only one unsaturated group, such as isopropene, butadiene andchloroprene, may be used as part of the monoethylenically unsaturatedmonomer content.

Polyethylenically unsaturated cross-linking monomers suitable forpreparing such polymer particles include diallyl phthalate, ethyleneglycol diacrylate, ethylene glycol dimethacrylate,trimethylolpropanetrimethacrylate, divinylsulfone; polyvinyl andpolyallyl ethers of ethylene glycol, of glycerol, of pentaerythritol, ofdiethyleneglycol, of monothio- and dithioderivatives of glycols, and ofresorcinol; divinylketone, divinylsulfide, allyl acrylate, diallylmaleate, diallyl fumarate, diallyl succinate, diallyl carbonate, diallylmalonate, diallyl oxalate, diallyl adipate, diallyl sebacate, divinylsebacate, diallyl tartrate, diallyl silicate, triallyl tricarballylate,triallyl aconitate, triallyl citrate, triallyl phosphate, divinylnaphthalene, divinylbenzene, trivinylbenzene; alkyldivinylbenzeneshaving from 1 to 4 alkyl groups of 1 to 2 carbon atoms substituted onthe benzene nucleus; alkyltrivinylbenzenes having 1 to 3 alkyl groups of1 to 2 carbon atoms substituted on the benzene nucleus;trivinylnaphthalenes, and polyvinylanthracenes.

The particularly preferred polymer system of the present invention isformed by the copolymerization of methylmethacrylate and ethylene glycoldimethylmethacrylate. Usually, the methylmethacrylate will be present atfrom about 10 to 80 percent of the monomer mixture, more usually atabout 20 to 70 percent of the monomer mixture, typically being in therange from about 25 to 65 percent of the monomer mixture, with theethylene glycol dimethylmethacrylate forming the remainder of themixture.

The preferred polymer particle matrix of the present invention comprisesrigid polymeric particles having a substantially non-collapsible porestructure. That is, the particles will substantially retain theirinternal pore structure even after the porogen (used in formation of theparticle as described hereinafter) has been extracted and the pores areempty. Such particles are mechanically stable compared with non-rigidmaterials, allowing manufacturing, processing, and handling of theparticles under relatively rigorous conditions which might result in therupture or damage of less stable materials. More importantly, thenon-collapsible pores facilitate formation of hard pellets or tabletsand introduction of the propellant or propellant solvent as described inmore detail hereinafter.

A polymeric particle of the present invention can be prepared bypolymerizing one or more polymers by a free radical suspensionpolymerization process. A monomer or pair of comonomers is dissolved inan inert porogen, which is also the active ingredient, to form asolution which is suspended in a phase or solvent incompatible with thesolution.

An example of a phase or solvent might be water with stabilizingadditives. After the solution is suspended in the phase, the solutionand phase are agitated to form a plurality of droplets of solutionsuspended in the phase. After the formation of the plurality of dropletsare activated to initiate a polymerization reaction in which a monomeris cross-linked or two or more monomers are polymerized to form porousparticles having a network of pores with the porogen held within thenetwork of pores. The activation may be triggered by an initiator whichis soluble in the monomer solution. Alternatively, activation may betriggered by an energy source such as radiation. The inert porogen willserve as an internal diluent during polymerization to introduce thedesired sponge-like macroporous structure or network of pores into thefinished particle. The inert porogen should not react with the monomerpresent during polymerization or inhibit the polymerization. After theformation of the porous particles, the particles are separated from thephase and may be subjected to one or more purification steps, such aswashing, to remove any unreacted monomer or impurity from the particles.Typically, the porogen is extracted and the propellant or otheradditives are introduced by absorption. In some cases, however, it maybe desirable to keep the porogen in the particles for use in the aerosoldispenser. For example, the porogen may be a solvent for the propellant.

The process of the present invention can be designed so as to controlporosity and the particle diameter of the particles. Under identicalpolymerization conditions, the porosity can be increased by increasingthe calculated or theoretical cross-linking density or by increasing theporogen concentration in the solution. An increase in porosity willincrease the surface area of the particle and hence the weight percentof the propellant or solvent that can be held within the particle.Particles having a porosity of greater than 30% are preferred and mostpreferably the porosity will be greater than 50%. In systems employing asolvent, the polymeric particle should be able to take up at least 100%and in some cases as much as 1500% of its own weight in solvent. Todecrease the particle diameter under identical polymerizationconditions, the agitation or the concentration of dispersion agents inthe phase should be increased. By controlling the particle diameter andparticularly the porosity of the particle, a material suitable for usein the method of the present invention can be obtained. Materialssuitable as porogens will be liquid substances which meet the followingcriteria:

1. They are either fully miscible with the monomer mixture or capable ofbeing made fully miscible by the addition of a minor amount ofnon-water-miscible solvent;

2. They are immiscible with water, or at most only slightly soluble;

3. They are inert with respect to the monomers, and stable when incontact with any polymerization catalyst used and when subjected to anyconditions needed to induce polymerization (such as temperature andradiation); and

4. They are readily extracted from the pore network of the particlesonce polymerization is complete.

Suitable porogens include a wide range of substances, notably inert,non-polar organic solvents. Some of the most convenient examples arealkanes, cycloalkanes, and aromatics. Specific examples of such solventsare alkanes of from 5 to 14 carbon atoms, straight or branched chaincycloalkanes of from 5 to 8 carbon atoms, benzene, and alkyl-substitutedbenzenes, such as toluene and the xylenes. For purposes of makingparticles having the high porosity necessary to be used with pressurizedpropellant systems, it has been found that cyclohexanol and particularlycyclohexanol combined with toluene or dodecanol are preferred porogens.

Extraction of the porogen may be effected by solvent extraction,evaporation, or similar conventional operations. The porogen extractionstep accomplishes the removal of unwanted species from the polymerizedstructures prior to impregnation with the desired active substance. Suchunwanted species include unreacted monomers, residual catalysts, andsurface active agents and/or dispersants remaining on the particlesurfaces.

Extraction of the porogen may be effected in a variety of ways,depending on the chemical nature of the porogen and its behavior incombination with that of the other species present. For example, theparticles may be recovered from the suspension by filtration, preferablyusing vacuum apparatus (such as a Beuchner funnel). The particles arethen washed with an appropriate solvent to remove organic species notbound to the polymer, including surfactants having deposited on theparticle surfaces from the aqueous phase, unreacted monomers andresidual catalysts, and the porogen itself. An example of such a solventis isopropanol, either alone or in aqueous solution. Once washing iscomplete, the solvent itself is removed by drying, preferably in avacuum.

In certain cases, an alternative method of extraction may be used--i.e.,where the porogen, unreacted monomer and water will form an azeotrope.In these cases, steam distillation is an effective way of extractingporogen from the particles. This again may be followed by drying undervacuum.

Once the particles are rendered dry and free of the porogen and anyunwanted organic materials, they may be pelletized and the propellantand, if desired, a solvent for the propellant are introduced to theinternal pore networks of the individual particles by absorption,typically in a suitable solvent. Such methods of introducing thepropellant will be described in more detail hereinbelow.

Pelletizing the Polymer

The polymeric materials of this invention are initially in the form oflight particles or beads which may be employed directly (withoutmodification) in the dispensing systems of the present invention.Unfortunately, small light materials such as these are difficult to addto containers during the manufacturing process (discussed above). Thus,to facilitate manufacturing of pressurized containers, the polymericparticles of this invention may be converted to heavier, rugged pelletsbefore being added to the containers. Delivering the polymer to theaerosol container in tablet form avoids the need for specialized systemsnecessary for coping with the air-borne particulate and solventsassociated with polymer powders or solutions. Thus, significantadvantages in the cost and convenience of the manufacturing process canbe realized.

The pellets formed according to this invention will have a hardness ofgreater than about 1 Kilogram as measured by a Penwalt Stokes tablethardness tester (Penwalt Stokes, Inc., Warminster, Pa.) and will resistcrushing, crumbling and fracture. Preferably, the pellets will be harderthan about 3 Kgs, and most preferably harder than about 4 Kgs. Thus,they can be safely transported and stored without break-up.

Pelleting or "tableting" can be achieved with or without a binder. Abinder may be used to facilitate agglomerating individual particlestogether to form a pellet. The binder will desirably also provideadditional hardness and crumble resistance to the pellet or tablet.Suitable binders include polyvinylpyrrolidone (PVP), polyvinyl alcohol(PVA), polyacrylic acid (PAA), hydrogenated cotton seed oil,hydroxymethyl cellulose, hydroxypropyl cellulose, methylcellulose,carboxymethyl cellulose, microcrystalline cellulose (such as Avicel),lignin sulfate, and other binders commonly used tableting andbriquetting operations. Magnesium stearate may also be added to thepolymeric material for the purpose of lubricating the material toprevent the pellets from sticking to the die surface.

The polymer particles to be pelletized are preferably pretreated totexturize them or render them "stickier", qualities which have beenfound to aid in agglomeration and pelletizing. One preferredpretreatment involves washing the polymers alternately in hot water andorganic solvent, e.g. acetone, as described in the examples below.Alternatively, unwashed polymer particles can be heated to drive off thevolatile porogen without removing the suspending agent such aspolyvinylpyrrolidone or polyvinyl alcohol, both of which can serve as abinder. Polymer particles pretreated by the above methods often formagglomerates even before pelletizing. These agglomorates may range insize from a few micrometers to 0.1 millimeter or more.

The tableting procedure may be accomplished with a wet or drygranulation process. In a preferred dry pelletizing procedure performedaccording to this invention, dry polymeric particles are mixed with abinder (and optionally magnesium stearate) to form a mixture havingabout 50-100% macroporous polymer, greater than 0% binder, and about0-15% magnesium stearate by weight. Preferably, the mixture will includeabout 80-100% macroporous polymer, about 0-20% binder, and about 0-5%magnesium stearate. Most preferably, the mixture will include greaterthan about 90% macroporous polymer, between about 0-10% binder, andbetween about 0-2% magnesium stearate. The resulting combination may betumbled on a roll-mill to assist in the mixing. After sufficient mixing,any large clumps of material are broken up and the particles are reducedto a relatively uniform size by passing the mixture through a screen forexample. A mesh screen of greater than about 25 mesh will work well.Preferably, the screen will be between about 10 and 21 mesh, with a mostpreferable screen being between about 14 and 20 mesh. The resultinggranular mixture is compressed in a standard tableting machine.

In a wet granulation procedure, the macroporous polymer is mixed withmagnesium stearate and either a dry binder or a wet binder solution. Ifa dry binder was employed, the mixture may be tumbled to achieve goodmixing, and water is then added to dampen the resulting mixture. At thispoint, either type of mixture may be reduced to granules of the propersize. A mesh screen of greater than about 25 mesh will work well.Preferably, the screen will be between about 10 and 21 mesh, with a mostpreferable screen being between about 14 and 20 mesh. Other means suchas extrusion of a thin strand followed by chopping can be employed toobtain granules of the proper size. The wet granules are next heated orotherwise treated to drive off most of the water. Finally, the driedgranules are compressed to hard tablets in a standard tableting machine.

Liquid Solvents

Liquid solvents may be used with the apparatus of the present inventionto dissolve additional propellant and thus enhance the sorption capacityof the macroporous polymeric matrix. Suitable solvents will generallydissolve for the propellant gas while being substantially insoluble ofthe polymeric material. Further, the solvent should not cause thesubstantially rigid macroporous polymer to swell. Acetone is aparticularly preferred solvent when carbon dioxide is used as thepropellant gas. Carbon dioxide may also be used with alcohol solventssuch as ethyl and methyl alcohol. Nitrous oxide will preferably be usedwith alcohol (ethyl or methyl) or ether solvents. Solvents for othergaseous propellants are well-known in the art. Suitable solvents mayalso include water, p-cymene, chloroform, benzene, toluene,acrylonitrile, nitrobenzene, o-dichlorobenzene, ethyl benzoate, methylmethacrylate, benzaldehyde, acetaldehyde, methyl benzoate, dimethylphthalate, furfural, aniline, butyrolactone, cyclohexanone, acetic acid,m-cresol, quinoline, acrylic acid, benzyl alcohol, propylene glycol andformamide.

While not wishing to be bound by theory, it is believed that the extentto which the polymeric material sorbs the liquid solvent (measured asvolume or weight of liquid per unit weight of polymeric material)corresponds to the potential gas storage performance of a givencombination of polymeric material and liquid solvent. While the use ofsubstantially pure solvents is envisioned, compatible mixtures of two ormore liquid solvents may be suitable for use in certain aspects of theinvention. Some such mixtures may be more practicable than puresolvents, as (for example) commercial ethanol often contains asubstantial percentage of water.

Minor quantities of impurities normally present in commercial-grade orindustrial-grade liquid solvents (as distinct from relatively purelaboratory-grade liquid solvents) are acceptable so long as they do notsignificantly or adversely affect the basic principles of the presentinvention in any of its aspects.

In addition to the above-mentioned functional requisites of atechnically suitable liquid solvent, regard should also be had to safetyfactors such as toxicity and environmental hazard. For such reasons,"benign" solvents such as water and lower alcohols (e.g. ethanol) arepreferred over known biohazards such as chlorinated hydrocarbons andbenzene, but in appropriate circumstances such considerations need notprevent adoption of liquid solvents that would be non-preferred in othercircumstances (particularly if containment was assured and recycling wasreliable). Other factors, such as economy and availability, may alsoinfluence a choice of liquid solvent or solvent mixture. cl PropellantGases

A wide variety of conventional propellant gases may be employed in thedispenser systems of the present invention, while environmentally benigngases such as carbon dioxide, nitrous oxide, nitrogen, oxygen, andmixtures of these such as nitrogen/oxygen mixtures including "natural"air are preferred, other conventional propellants may be adopted, forexample ammonia or sulphur dioxide. The present invention may even beused with problematic gases, such as freon and hydrocarbons, althoughsuch use is not preferred for the reasons stated above. Minor quantitiesof impurities normally present in commercial-grade or industrial-gradegases (as distinct from relatively pure laboratory-grace gases) may bepresent in the propellant(s) so long as they do not significantly oradversely affect the basic principles of the present invention in any ofits aspects.

The propellant gas may be converted to a cryogenically cool=d liquid orsolid such as "dry ice" in the case of carbon dioxide. Where thepropellant gas is solidified, the solidified gas is preferablypelletized or in particulate form for greater ease of separating andmetering the substantially predetermined amount of propellant gas from abulk supply thereof. The polymeric material and solvent (when applied)may also be pelletized or in particulate form for greater ease ofseparating and metering the substantially predetermined quantity thereofinto the pressurizable container.

Products to be Dispensed

In general, any substance which is dispensable from a pressurizedcontainer is suitable for use with the present invention, subject tosuch practical limitations as the compatibility of the product with thepropellant in non-barrier and semi-permeable barrier systems. Suitablesubstances for dispensing from a pressure pack dispenser includelubricant compositions, anti-corrosion agents, de-icers, sealingcompounds, paints, insecticides, polishes, cosmetics, shaving cream,non-stick cooking products, hair spray, cleaning materials andpharmaceutical substances.

In some instances, the propellant will be dispensed with the product andmay even be necessary to impart a particular consistency or form to theproduct. For example, in some lubricants and hygiene products, thecombination of the dispenser and the propellant system functions as afoam generator.

It is also within the scope of the present invention that the propellantgas constitutes part of the dispensed product for example as inflationgas for inflating articles such as tires or balloons, as gaseous fuel oroxidizer in combustion, cutting, or welding systems, as a breathing gasor breathing gas mixture, and as an industrial or laboratory gas.

The invention will be further illustrated in the examples that followwherein the polymeric material is a copolymer of methyl methacrylate andethylene glycol dimethacrylate.

Experimental Example 1: Methyl Methacrylate High Acetone AbsorptionPolymers, 20% Cross-linking and Use Dodecanol and Cyclohexanol asPorogens

29.03 g of dodecanol and 240.97 g of cyclohexanol were melted in abeaker at 60° C. to form a porogen mixture. Then 0.45 g of AIBN (2,2azobisisobutyronitrite) and a mixture of 24 g of methyl methacrylate and6.0 g of ethylene glycol dimethacrylate were dissolved in the porogenmixture. The resulting solution was combined with a mixture of 9.00 g ofa 20% aqueous solution of polyvinylpyrrolidone (MW 700,000 PVP K-90 fromGAF Chemical Corp. of Wayne, N.J.) and 600 g of deionized water in avessel provided with a agitator, a thermometer, a nitrogen inlet andreflux condenser. To avoid clumping, the temperature was slowlyincreased from room temperature to 60° C. over 30 minutes. Theexothermic polymerization starts at about 57° C. as indicated by atemperature increase. After one hour, the nitrogen gas supply wasdisconnected. Polymerization was carried out at about 75° C. for 8 hoursat a stirring speed of 730 rpm. When the reaction was completed, theproduct was filtered with hot water to remove the PVP K-90. Then thepolymer washed alternately with hot water and acetone until the filtratewas colorless. The resulting polymers were dried in a vacuum oven at 80°C. for 8 hours.

Example 2: Methyl Methacrylate High Acetone Absorption Polymers, 40%Cross-Linking and Use Dodecanol and Cyclohexanol as Porogens

29.03 g of dodecanol and 240.97 g of cyclohexanol were melted in abeaker at 60° C. to form a porogen mixture. Then 0.45 g of AIBN (2,2azobisisobutyronitrite) and a mixture of 18 g of methyl methacrylate and12 g of ethylene glycol dimethacrylate were dissolved in the porogenmixture. The resulting solution was combined with a mixture of 6.75 g ofa 20% aqueous solution of polyvinylpyrrolidone (MW 700,000 PVP K-90 fromGAF Chemical Corp. of Wayne, N.J.) and 450 g of deionized water in avessel provided with a agitator, a thermometer, a nitrogen inlet andreflux condenser. To avoid clumping, the temperature was slowlyincreased from room temperature to 60° C. over 30 minutes. Thepolymerization starts at about 57° C. After one hour, the nitrogen gassupply was disconnected. Polymerization was carried out at about 75° C.for 8 hours at a stirring speed of 730 rpm. When the reaction wascompleted, the product was filtered with hot water to remove the PVPK-90. Then the polymer washed alternately with hot water and acetoneuntil the filtrate was colorless. The resulting polymers were dried in avacuum oven at 80° C. for 8 hours.

Example 3: Methyl Methacrylate High Acetone Absorption Polymers, 40%Cross-Linking and Use Toluene and Cyclohexanol as Porogens

27 g of toluene and 243 g of cyclohexanol were stirred in a beaker untilthe cyclohexanol was completely dissolved. Then 0.45 g of AIBN (2,2azobisisobutyronitrite) and a mixture of 18 g of methyl methacrylate and12 g of ethylene glycol dimethacrylate were added to thetoluene-cyclohexanol mixture. The resulting solution was combined with amixture of 9.00 g of a 20% aqueous solution of polyvinylpyrrolidone (MW700,000 PVP K-90 from GAF Chemical Corp. of Wayne, N.J.) and 600 g ofdeionized water in a vessel provided with a agitator, a thermometer, anitrogen inlet and reflux condenser. To avoid clumping, the temperaturewas slowly increased from room temperature to 60° C. over 30 minutes.The polymerization starts at about 57° C. After one hour, the nitrogengas supply was disconnected. Polymerization was carried out at about 75°C. for 8 hours at a stirring speed of 730 rpm. When the reaction wascompleted, the product was filtered with hot water to remove the PVPK-90. Then the polymer washed alternately with hot water and acetoneuntil the filtrate was colorless. The resulting polymers were dried in avacuum oven at 80° C. for 8 hours.

Example 4: Methyl Methacrylate High Acetone Absorption Polymers, 70%Cross-Linking and Use Toluene and Cyclohexanol as Porogens

54 g of toluene and 486 g of cyclohexanol were stirred in a beaker untilthe cyclohexanol was completely dissolved. Then 0.90 g of AIBN (2,2azobisisobutyronitrite) and a mixture of 18 g of methyl methacrylate and42 g of ethylene glycol dimethacrylate were added to thetoluene-cyclohexanol mixture. The resulting solution was combined with amixture of 9.00 g of a 20% aqueous solution of polyvinylpyrrolidone (MW700,000 PVP K-90 from GAF Chemical Corp. of Wayne, N.J.) and 600 g ofdeionized water in a vessel provided with a agitator, a thermometer, anitrogen inlet and reflux condenser. To avoid clumping, the temperaturewas slowly increased from room temperature to 60° C. over 30 minutes.The polymerization starts at about 57° C. After one hour, the nitrogengas supply was disconnected. Polymerization was carried out at about 75°C. for 8 hours at a stirring speed of 850 rpm. When the reaction wascompleted, the product was filtered with hot water to remove the PVPK-90. Then the polymer washed alternately with hot water and acetoneuntil the filtrate was colorless. The resulting polymers were dried in avacuum oven at 80° C. for 8 hours.

Example 5: Methyl Methacrylate High Acetone Absorption Polymers, ReplacePVP K-90 with PVOH

29.03 g of dodecanol and 240.97 g of cyclohexanol were melted in abeaker at 60° C. to form a porogen mixture. Then 0.45 g of AIBN (2,2azobisisobutyronitrite) and a mixture of 24 g of methyl methacrylate and6.0 g of ethylene glycol dimethacrylate were dissolved in the porogenmixture. The resulting solution was combined with a mixture of 7.5 g ofpolyvinylalcohol (MW 78,000, 88% hydrolyzed) and 300 g of deionizedwater in a vessel provided with a agitator, a thermometer, a nitrogeninlet and reflux condenser. To avoid clumping, the temperature wasslowly increased from room temperature to 60° C. over 30 minutes. Thepolymerization starts at about 57° C. After one hour, the nitrogen gassupply was disconnected. Polymerization was carried out at about 75° C.for 8 hours at stirring speed of 730 rpm. When the reaction wascompleted, the product was filtered with hot water to remove thepolyvinylalcohol. Then the polymer washed alternately with hot water andacetone until the filtrate was colorless. The resulting polymers weredried in a vacuum oven at 80° C. for 8 hours.

Example 6: Methyl Methacrylate High Acetone Absorption Polymers, WithoutSuspending Agent Present in the Aqueous Phase

29.03 g of dodecanol and 240.97 g of cyclohexanol were melted in abeaker at 60° C. to form a porogen mixture. Then 0.45 g of AIBN (2,2azobisisobutyronitrite) and a mixture of 24 g of methyl methacrylate and6.0 g of ethylene glycol dimethacrylate were dissolved in the porogenmixture. The resulting solution was mixed 300 g of deionized water in avessel provided with a agitator, a thermometer, a nitrogen inlet andreflux condenser. To avoid clumping, the temperature was slowlyincreased from room temperature to 60° C. over 30 minutes. Thepolymerization starts at about 57° C. After one hour, the nitrogen gassupply was disconnected. Polymerization was carried out at about 75° C.for 8 hours at a stirring speed of 730 rpm. When the reaction wascompleted, the product was filtered with hot water. Then the polymerwashed alternately with hot water and acetone until the filtrate wascolorless. The resulting polymers were dried in a vacuum oven at 80° C.for 8 hours.

Example 7: 70% Cross-Linking with Dodecanol and Cyclohexanol as thePorogen

29.03 g of dodecanol and 240.97 g of cyclohexanol were melted in abeaker at 60° C. to form a porogen mixture. Then 0.45 g of AIBN and amixture of 9 g of methyl methacrylate and 21 g of ethylene glycoldimethacrylate were dissolved in the porogen mixture. The resultingsolution was combined with a mixture of 6.75 g of polyvinylpyrrolidone(PVP K-90) and 450 g of deionized water. Polymerization and treatment ofthe polymer was carried out as described in Example 1.

Example 8: Changing the Porogen to Monomer Ratio from 90/10 to 70/30

7.53 g of dodecanol and 62.47 g of cyclohexanol were melted in a beakerbefore being dissolved in 0.3 g of AIBN and a mixture of 18 g of methylmethacrylate and 12 g of ethylene glycol dimethacrylate. The resultingsolution was then added to 180 g of deionized water containing 2.7 g of20% aqueous solution of polyvinylpyrrolidone (PVP K-90). Thepolymerization and treatment of the polymer was carried out as describedin Example 1.

Example 9

A mixture of 2017.5 grams of toluene and 1788 grams of cyclohexanol wasused as a porogen. 16.8 grams of benzoyl peroxide, 201.7 grams of methylmethacrylate and 470 grams of ethylene glycol dimethacrylate were addedto the porogen. The resulting solution was combined with a mixture of56.3 grams of methylcellulose ("Methocel", Dow Chemical Co., Midland,Mich.) and 5377 grams of deionized water. Polymerization and treatmentof the polymer was carried out as described in Example 1.

Example 10: Dry Granulation

Polymer particles produced as described in Example 1 were mixed with aninert binder of hydrogenated cottonseed oil (Tradename Duratex) in aratio of 9 parts polymer to 1 part Duratex. Magnesium stearate was addedto the mixture at a level of 1% of the total weight of the mixture. Themixture was then screened to remove lumps.

Tablets were made on a standard tableting machine (Penwalt Stokes, Inc.,Warminster, Pa). The resulting tablets had diameters ranging from 15 mmto 9 mm and weights ranging from 0.3 grams to 0.6 grams.

The tablets produced exhibited hardness ranging from 7.5 kgs to 13 kgswhen the hardness was measured by a Penwalt Stokes tablet hardnesstester.

The acetone pick-up of the tablets were measured and compared to theuntableted polymer containing no binder or magnesium stearate.

    ______________________________________                 Acetone pick-up/gm of                 polymer or tablet    ______________________________________    Polymer tablet  9 gms    Untableted Polymer                   10 gms    ______________________________________

Example 11: Dry Granulation

Polymer particles made as described in Example 2 were blended with amicrocrystalline cellulose (tradename Avicel) and magnesium stearate inweight ratios of 90 parts polymer 10 parts avicel and 1 part magnesiumstearate. The blend was tumbled on a roll-mill to assist mixing of theblend.

0.30 gm tablets were made from the blend. The tablets exhibited ahardness of 9.5 kgs. The acetone pick-up of the untableted polymer andthe tablets was as follows:

    ______________________________________                 Acetone pick-up/gm of                 polymer or tablet    ______________________________________    Polymer tablet 10.6 gms    Untableted Polymer                    9.7 gms    ______________________________________

Example 12: Wet Granulation

Polymer particles made as described in Example 3 were blended withpolyvinyl alcohol (PVOH) and magnesium stearate in the proportions of 90gms polymer, 10 gms PVOH and 1 gm of magnesium stearate. Enough waterwas added to the blend to facilitate uniform mixing of the components ofthe blend and to achieve a damp mixture in which no excess water couldbe squeezed from the mixture.

The wet mixture was passed through a 20 mesh screen to form granules.The granules were dried in an oven at 70° C. for 8 hours.

The dry granules were tableted to produce 0.35 gm tablets havinghardness of 9.5 kgs.

Acetone pick-up after tableting was 8.5 gms per tablet compared to 9.5gms per gram of untableted polymer.

Example 13: Wet Granulation

100 grams of polymer particles from Example 4 was mixed with 30 grams ofpowdered polyvinylpyrrolidone and 1 gram of magnesium stearate. Theblend was tumbled to achieve good mixing. 100 cc of water was added tothe mixture and mixed in. The mixture was passed through a 14 meshscreen. The granules thus formed were dried in an oven at 60° C. for 10hours. The granules were formed into tablets, each having a weight of0.5 grams.

Example 14: Wet Granulation

100 grams of polymer particles as produced in Example 5 was mixed with 1gram magnesium stearate and 300 grams of a 10% solution methyl cellulose(Methocel). The mixture was blended together, passed through a 20 meshscreen and the granules dried in an oven at 70° C. overnight. The driedgranules were made into 0.5 gm tablets.

The tablets were evaluated for hardness and its ability to withstandabuse by attempting to crush a tablet between the thumb, index fingerand middle finger of one hand, and by allowing the tablet to fall to thefloor from a height of 8 feet. The tablets were strong and rugged enoughto withstand such abuse.

Discharge Characteristics

FIG. 2 shows the discharge curve for a pressurized, vapor-tightcontainer charged with 1.13 g carbon dioxide, 8.4 ml acetone and 1.53 gpolymer produced as in Example 2. A bag-type dispenser (as describedabove) outfitted with a pressure gauge was used to make themeasurements. As shown, the pressure is initially near 100 psi and dropsoff gradually as the container is evacuated until a final pressure ofabout 38 psi is reached. The polymers employed in this test had aporosity of greater than 3.0 cc/gram.

As a further example, a polymeric material as produced in Example 9 wastested in the same vapor-tight container. The polymers employed in thistest had a porosity of greater than 3.0 cc/gram. 1.11 g polymer, 8.4 mlacetone and 1.11 g carbon dioxide were added to the container and theresulting discharge curve is shown in FIG. 3. The end pressure taken 5minutes after extrusion was complete was 39 psi.

FIG. 4 shows the discharge curve for a pressurized, vapor-tight bag-typecontainer charged with carbon dioxide only. No macroporous polymer wasemployed. As shown, the pressure is initially near 105 psi and drops offgradually as the container is evacuated until a final pressure of lessthan 20 psi is reached.

While certain modifications and variations have been described above theinvention is not restricted thereto, and other modifications andvariations can be adopted without departing from the scope of theinvention as defined in the appended claims.

What is claimed is:
 1. A process for preparing a pellet for use inpressurized dispensing containers, said method comprising the followingsteps:mixing substantially rigid crosslinked polymer macroporousparticles with a binder to form a mixture of polymer macroporousparticles and binder; and compressing the mixture of polymer macroporousparticles to form a pellet, wherein said polymer macroporous particlesdefine a substantially non-collapsible pore network and said pelletdefines interstitial spaces between said polymer macroporous particles,and wherein the interstitial spaces are substantially larger than thepore network.
 2. A process as recited in claim 1 wherein the particleshave a particle size of between about 1 and about 500 μm; a particledensity of between about 0.1 and about 2.0 g/cc; a pore volume ofbetween about 0.5 and about 6.0 cc/g; an average pore diameter ofbetween about 0.001 and about 3 μm; and a surface area of between about1 and about 500 m² /g.
 3. A process as recited in claim 1 wherein thebinder is selected from a group consisting of hydrogenated cottonseedoil, polyvinyl alcohol, polyvinylpyrrolidone, and microcrystallinecellulose.
 4. A process as recited in claim 1 wherein said particles arecomposed of a copolymer of methacrylate and ethylene glycoldimethylmethacrylate.
 5. A process as recited in claim 1 wherein theparticles range from about 10 mesh to about 21 mesh.
 6. A process asrecited in claim 1 wherein said polymer particles are texturized by apretreatment, said pretreatment comprising the following steps:washingthe particles with hot water, and washing the particles with acetone. 7.A process as recited in claim 1 wherein monomers are polymerized in anorganic porogen suspended in an aqueous phase to form said polymericparticles, wherein said polymer particles are pretreated by heating toremove substantially all of said porogen.